The present invention relates to a system and a method for controlling ignition timing of an engine for a motor vehicle.
In a fuel injection control system having an injection pulse width calculator based on the quantity of intake air, the intake air quantity must be measured with precision. As an intake air quantity sensor, an airflow meter with a hot film type or with a hot wire is provided in an intake passage at a position upstream of a throttle valve of an engine to detect the quantity of the intake air.
Since the sensor has a high response, the output of the sensor oscillates as shown by a dot-dash line of FIG. 6 because of the pulsations of intake air induced in a cylinder of the engine. Heretofore, the output Qs is averaged to obtain the intake air quantity Qs'.
In the fuel injection control system, a basic fuel injection pulse width Tp is determined in accordance with the intake air quantity Qs' and engine speed N as follows. EQU Tp=K.multidot.Qs'/N (K:constant)
An actual fuel injection pulse width Ti is obtained by correcting the basic fuel injection pulse width Tp with various coefficients such as a coolant temperature coefficient, an acceleration coefficient, and a feedback correcting coefficient, so that the air fuel mixture is prevented from becoming rich or lean.
In an ignition timing control system, the basic fuel injection pulse width Tp obtained based on the average quantity Qs' is regarded as engine load. An actual ignition timing is derived from an ignition timing map in accordance with the basic injection pulse width Tp and the engine speed N.
When the throttle valve is rapidly opened for accelerating the engine, the intake air quantity sensor detects the amount of intake air Qs including intake air induced in the cylinders of the engine and intake air induced in an air chamber downstream of the throttle valve and an intake manifold.
In other words, all the air passing the throttle valve is measured by the sensor. Accordingly, the air actually induced in the cylinders can not be measured at once. The actual quantity induced in the cylinders appears on the output of the sensor with a delay D as shown in FIG. 6.
In a single-point injection system, an injector is disposed upstream of the throttle valve. Accordingly, the quantity of injected fuel can be properly determined in accordance with the quantity of intake air passing through the throttle valve. However, it is necessary to determine the ignition timing of the engine based on the quantity of intake air actually induced in the cylinders. Accordingly, if the ignition timing is determined in accordance with the basic fuel injection pulse width Tp as a parameter, ignition is temporarily delayed in the transient state. As a result, the power of the engine is temporarily reduced to reduce the driveability of the motor vehicle. Similarly, when the throttle valve is rapidly closed, the air-fuel ratio deviates to aggravate the emission.
Japanese Patent Application Laid Open Nos. 61-229954, and 62-265449 disclose systems for controlling ignition timing, where the quantity of intake air actually induced in the cylinder is estimated from an electric equivalent circuit. However, it is difficult to estimate the quantity of air in the transient state, and hence the ignition timing can not be accurately controlled.
Japanese Patent Application Laid Open No. 62-261645 describes a method for estimating the actual quantity of intake air induced into the cylinder. In this method, the pressure of the intake air downstream of the throttle valve is obtained as a function of throttle opening degree and the quantity of air detected by an air flow meter under atmospheric pressure. The quantity of air charged in a chamber and an intake passage downstream of the throttle valve in the transient state is calculated from differentiation of the calculated pressure with respect to time and the volume of the chamber and the intake passage. The actual intake quantity is obtained by subtracting the quantity of air charged in the intake manifold and the air chamber from the intake air quantity detected by the air flow meter.
However, since the actual quantity of air induced into the cylinders of the engine is only an estimation from the throttle position, the ignition timing can not be properly controlled.